As is known per se, circularly symmetrical objects are printed in a screen-printing machine by turning the object about its own longitudinal axis and by concomitantly moving a screen in a plane tangential to the object, with ink being transferred through the screen by a squeegee being pressed against said screen, the screen being squeezed between the object and the squeegee.
Flat objects such as compact disks can also be printed by screen-printing. They are then held stationary under the screen which is also stationary, and the squeegee is moved over the length of the-screen to transfer the ink progressively to the flat object.
Such machines are used for repetitively printing identical objects.
Solutions have been proposed in order to automate putting the objects in place in the print station and removing them therefrom. In such machines, the objects to be printed are brought into the vicinity of the print station by a feed conveyor, and they are then transferred from the conveyor to the print station by a transfer device. In addition, a symmetrical other transfer device transfers the printed objects from the print station to a removal conveyor.
On the conveyors, the objects to be printed are generally disposed with an orientation different from the orientation of the objects in the print station.
More precisely, when the objects to be printed are flasks, they are placed vertically on the surface of the conveyor with their longitudinal axes disposed perpendicularly to the surface of the conveyor. Whereas, in the print station, the objects must be disposed with their longitudinal axes disposed generally horizontally, i.e. perpendicularly to the initial orientation direction of the objects on the conveyor.
Therefore, the transfer device interposed between the feed conveyor and the print station is suitable for moving the object all the way between the outlet end of the conveyor and the print station while also simultaneously turning the object over.
The opposite movement must be provided by the other transfer device in order to transfer the printed object from the print station to the removal conveyor.
Such loading and/or unloading handling devices are described, for example, in Documents FR-2 775 472 and U.S. Pat. No. 4,907,504.
In those documents, the transfer device comprises a manipulator arm equipped with an end clamp. The arm is hinged relative to the frame of the print station to pivot about an axis extending exactly perpendicularly to or parallel to the support surface for supporting the objects on the feed conveyor. The clamp is hinged relative to the arm so that the arm and the clamp are moved concomitantly.
With such a device, it is observed that, for objects whose bases are relatively wide, the periphery of the base hits the surface of the conveyor during the initial stage in which the object is moved towards the print station. Tilting the object by turning the clamp starts while the object is still in contact with the conveyor. Thus, the periphery of the base of the object exerts a pressure on the conveyor, thereby generating large forces in the transfer device and in the conveyor, reducing their lives.
Likewise, it is observed that such forces exist while the objects are being put in place in the print station. In addition, excessive forces are also encountered by the transfer device for removing the printed objects and transferring them to the removal conveyor.